A common method for measuring the blood pressure of a patient employs an inflatable cuff (sometimes referred to as a Riva-Rocci cuff) wrapped around the upper arm of the patient whose blood pressure is to be determined. As the cuff is inflated, the cuff pressure, and the pressure applied to the arm of the patient, increases. As the pressure applied to the arm is increased beyond the highest blood pressure level in a brachial artery located in the arm beneath the cuff, the artery is forced closed.
The blood pressure in the brachial artery is not constant, but varies with time in relation to the beating of the heart. Following a contraction of the heart to pump blood through the circulatory system, the blood pressure first increases to a maximum level, then reduces to a minimum level. The maximum blood pressure level between heart beats is known as the systolic blood pressure level. The minimum blood pressure level between heartbeats is known as the diastolic blood pressure level.
As the pressure in the inflatable cuff is reduced from a high level above the systolic blood pressure level, where the brachial artery is forced permanently closed, to a pressure level below the systolic blood pressure level, the brachial artery beneath the cuff will begin to open and close with each heart beat as the blood pressure level first exceeds the cuff pressure, and then falls below the cuff pressure. The arterial wall acts in a non-linear fashion with respect to the blood pressure level. Thus, as the blood pressure exceeds the cuff pressure, the artery will literally "snap" open, producing a low frequency blood pressure sound. This blood pressure sound may be detected using a microphone placed under the cuff against the patient's arm. The turbulent flow of blood through the artery following the opening snap also produces sounds, known as Korotkoff sounds, that may be detected using a stethoscope or microphone placed near the down-stream end of the cuff on the patient's arm. The highest cuff pressure at which Korotkoff sounds or blood pressure sounds are detectible thus corresponds to the systolic blood pressure level of the patient.
As the cuff pressure is gradually reduced further, the cuff pressure will be brought below the diastolic blood pressure level through a gradual pressure change. At this pressure level, the brachial artery beneath the cuff remains open throughout the heartbeat cycle. Blood pressure sounds, caused by the opening of the artery, and Korotkoff sounds, will, therefore, not be produced. The lowest cuff pressure at which blood pressure sounds or Korotkoff sounds can be detected thus corresponds to the diastolic blood pressure level of the patient. The determination of blood pressure levels in this manner, based on the detection of the onset and disappearance of Korotkoff sounds or blood pressure sounds as varying pressures are applied to an artery, is known as an auscultatory method of blood pressure determination.
In manual auscultatory blood pressure measurement methods, a stethoscope is used to detect the onset and disappearance of Korotkoff sounds. Thus, manual blood pressure measurements are highly dependent on the skill and hearing ability of the person taking the measurement. To overcome this dependence on human skill and judgment, and to automate the process of determining a patient's blood pressure, automatic blood pressure monitoring systems based on the auscultatory method of blood pressure determination have been developed. These automatic systems employ one or more microphones placed in or under an automatically inflatable and deflatable cuff to detect blood pressure sounds. However, movement of the patient, such as during exercise, vibration during transport, and other activity around the patient, will cause noise to be picked up by the blood pressure monitor microphones along with the blood pressure sounds. The automatic blood pressure monitoring system must, therefore, be able to separate the noise from the blood pressure sounds in order to accurately determine the patient's blood pressure levels. This has been achieved, for example, by filtering the microphone signal using a band pass filter whose pass band corresponds to a known frequency range of blood pressure sounds. This eliminates much of the noise from the microphone signal.
Some automatic blood pressure monitoring systems employ two microphones for detecting blood pressure sounds. For example, two microphones may be placed under the inflatable cuff separated by a distance such that a low frequency blood pressure sound will reach the first microphone approximately 180.degree., but at least more than 90.degree., out of phase from the second microphone. Noise signals will tend to reach each microphone essentially simultaneously, and in phase. Therefore, subtracting the two microphone signals from each other will tend to enhance the useful data and diminish unwanted noise. The two microphone signals can be added and subtracted from each other to create signal and noise detection thresholds. Microphone signals are considered to be valid blood pressure sound detections if they meet the detection thresholds.
Another method that makes use of two microphones relies on using a first microphone placed on the arm of the patient upstream from a second microphone to provide a time gate for the second microphone. Once again, this method relies on the fact that blood pressure sounds will propagate down the arm of the patient from the first microphone to the second microphone, whereas a noise signal will likely be picked up by both microphones simultaneously. When a sound signal exceeding a certain threshold is picked up by the first microphone, a time gate is opened which causes the downstream microphone to "listen" during a time interval a few milliseconds later, corresponding to the estimated propagation time of a blood pressure sound between the two microphones. If a sound signal is picked up by the second microphone during the gate period, the signal may be considered a valid blood pressure sound detection. A similar gating scheme may be used for blood pressure monitors employing a single microphone. Since the time delay between the contraction of the heart and the appearance of a blood pressure sound at a microphone placed on the arm of a patient can be estimated, the signal from a ECG heart monitor may be used to generate a delayed gate for the blood pressure monitor microphone. A sound signal picked up by the microphone during the gate period may be considered a valid blood pressure sound detection.
An effective automatic blood pressure monitoring system and method is described in pending U.S. patent application Ser. No. 08/665,286, entitled "Method and Apparatus for Detecting Blood Pressure by Blood Pressure Sounds in the Presence of Significant Noise", filed Jun. 17, 1996, by the inventors of the present invention. This method for accurately detecting the blood pressure sounds produced by the opening snap of a patient's artery in the presence of significant noise levels uses the phase information contained in two microphone signals. Two microphones are placed on a patient along the axis of an artery, with their centers separated by a distance such that blood pressure sounds picked up by each of the microphones will be approximately 180.degree., but at least more than 90.degree., out of phase with each other. Noise picked up by the microphones will typically be in phase. The two microphone signals are filtered using band pass filters having pass bands corresponding to the frequency range of the blood pressure sounds. This removes some of the noise from the microphone signals. The filtered microphone signals are then sampled and multiplied together, or convolved in the frequency domain, to generate a microphone signal product. If the microphone signal product is negative, indicating that the microphone signals are out of phase at the sample time, the detection of a valid blood pressure signal for that sampling time is indicated. A selected number of consecutive valid blood pressure signal detections indicates the detection of a blood pressure sound. The detection of blood pressure sounds at a range of cuff pressures is used to determine the systolic and diastolic blood pressure levels of the patient. This blood pressure monitoring method enables accurate blood pressure measurements to be made during the extremes of noise interference encountered during patient movement, shivering, or exercise, or caused by ambient vibrations encountered in highly noisy environments. However, even this highly effective method for monitoring blood pressure may be improved. Since this method relies on the phase information, rather than the amplitude information, contained in the two microphone signals, a false blood pressure sound detection may be indicated in certain situations. For example, if two independent noise signals are separated in time by an amount corresponding to the expected phase difference of the blood pressure sounds, the product of the two microphone signals will be negative, and the detection of a blood pressure sound may be erroneously indicated. This may occur even if the amplitude of the noise signals is relatively low.